1. DSPMC/ip. I use it and love it although it is a bit pricey it does handle the analog drives nicely and reads the encoders back into Mach3. It also uses 24v I/O.
2. I Don't know.
3. I am using the DSPMC and Mach3 on my BTC-1 with amt-102 encoders.
4. As long as your encoders are 5 volt I don't see why not. The DSPMC will handle very high encoder counts and fast speeds so no may be the time to upgrade the encoders. Of course if your encoders are 1000 count or better they should be just fine.
I hope this helps, Mike